Effects of diffusion current on characteristics of metal-oxide (insulator)-semiconductor transistors

Abstract

A qualitative discussion of the device operation is first given using three-dimensional energy band diagrams to show the significance of the diffusion current. The theoretical static I–V characteristics are the computed including both the diffusion and the drift currents, based on the one-dimensional and gradual channel model. Drain current saturation phenomena are evident in these exact solutions which are in good agreement with the calculations based on the bulk charge approximation and with the experimental data for the entire non-saturating and saturated ranges. The relative importance of the two current components along the length of the channel is illustrated. The effects of the diffusion current on the three more important low-frequency dynamic characteristics (the short-circuit gate capacitance, the transconductance, and the drain conductance) are discussed. The surface potential, the quasi-Fermi potential, the surface electric field and the surface carrier concentration along the channel are examined. The complete one-dimensional gradual channel model is inadequate to account for the large drain conductance observed in the saturation range, and it is shown that the electric field longitudinal to the channel current flow must be taken into account near the drain junction where it is larger than the transverse field due to the voltage applied to the gate electrode.